Infrared Spectra of M (OH) 1, 2, 4 (M= Pb, Sn) in Solid Argon

Infrared absorptions for the matrix-isolated lead and tin hydroxides M(OH), M(OH)2 and M(OH)4 (M = Pb, Sn) were observed in laser-ablated metal atom ...
1 downloads 0 Views 148KB Size
Download PDF
J. Phys. Chem. A 2005, 109, 9013-9020

9013

Infrared Spectra of M(OH)1,2,4 (M ) Pb, Sn) in Solid Argon Xuefeng Wang and Lester Andrews* Chemistry Department, UniVersity of Virginia, CharlottesVille, Virginia 22904-4319 ReceiVed: June 23, 2005; In Final Form: August 10, 2005

Infrared absorptions for the matrix-isolated lead and tin hydroxides M(OH), M(OH)2 and M(OH)4 (M ) Pb, Sn) were observed in laser-ablated metal atom reactions with H2O2 during condensation in excess argon. The major M(OH)2 product was also observed with H2 and O2 mixtures, which allowed the substitution of 18O2. The band assignments were confirmed by appropriate D2O2, D2, 16O18O, and 18O2 isotopic shifts. MP2 and B3LYP calculations were performed to obtain molecular structures and to reproduce the infrared spectra. The minimum energy structure found for M(OH)2 has Cs symmetry and a weak intramolecular hydrogen bond. In experiments with Sn, HD, and O2, the internal D bond is favored over the H bond for Sn(OH)(OD). The Pb(OH)4 and Sn(OH)4 molecules are calculated to have S4 symmetry and substantial covalent character.

Introduction Lead is one of the most common dispersed heavy metals in the atmosphere in the modern industrial era.1-3 The toxicity on living organisms has been confirmed as displacement of vital metals in the organism leading to anemia and damage to the nervous system.4,5 The PbOH radical might be produced in the atmosphere, but no definitive experimental observation has been made based on our knowledge.6 However, two weak, broad near-infrared emission bands in a study of lead monohalides have been tentatively assigned to HPbO (or PbOH).7 The reactions of lead and tin with H2 and O2 in solid matrices have been studied separately, and the MO, MO2, MH2, and MH4 molecules have been identified through infrared spectra.8,9 The analogous SnX2, SnX4, and PbX2 (X ) Cl, Br, I) molecules have been observed by electron diffraction.10 However, tetravalent lead is not stable since relativistic effects reduce the M-X bond strength, so only PbCl4 has been observed experimentally.10,11 It is very interesting to consider the lead and tin hydroxide molecules and their bonding and structure since very little is known about group 14 metal hydroxides.12a Solubility product constant data indicate that Sn(OH)2 and Pb(OH)2 are very weak bases, much weaker than Mg(OH)2.12b Recently we developed a new method to investigate metal hydroxide molecules using laser-ablated metal atom reactions with H2O2 or H2 + O2 mixtures in low-temperature matrices.13 With this approach we have the advantage to identify most infrared active modes without overlapping excess water absorptions. For example, ionic M(OH)2 (M ) group 2 metals), ionic M(OH)2 and M(OH)4 (M ) group 4 metals), and covalent M(OH)2 molecules (M ) group 12 metals) have been investigated. We present here a matrix infrared spectroscopic study of laser-ablated tin and lead atom reactions with H2O2 and H2 + O2 in solid argon and identify the new metal hydroxide molecules M(OH), M(OH)2 and M(OH)4 (M ) Sn, Pb). Experimental and Computational Methods Laser-ablated Sn and Pb atom reactions with oxygen and with hydrogen in excess argon at 10 K have been described in our previous papers.9 The Nd:YAG laser fundamental (1064 nm, 10 Hz repetition rate with 10 ns pulse width) was focused onto * Corresponding author. E-mail: [email protected].

the rotating high purity metal targets (Johnson Matthey), which gave bright plumes spreading uniformly to the cold CsI window. The tin and lead targets were polished to remove the oxide coating and immediately placed in the vacuum chamber. The laser energy was varied about 10-20 mJ/pulse. Since the tin and lead targets eroded rapidly during the ablation because of metal softness, the targets were adjusted every 5 min to get a fresh surface. FTIR spectra were recorded at 0.5 cm-1 resolution on Nicolet 750 with 0.1 cm-1 accuracy using an MCTB detector. Matrix samples were annealed at different temperatures, and selected samples were irradiated by a medium-pressure mercury arc lamp (Philips, 175 W) with the globe removed.14 Urea hydrogen peroxide (UHP) (Aldrich, 98%) was used as the H2O2 source.13,15 At room-temperature UHP was put into a Chemglass high vacuum valve with Teflon stem and argon gas was passed over the sample to provide H2O2. Deuterated ureaD2O2 was prepared following previous workers.16 Samples of H2, D2, HD, O2, 18O2, and 16,18O2 were diluted in argon as received. Complementary MP2 calculations were performed using the Gaussian 98 program system,14 the 6-311++G(3df,3pd) basis set for hydrogen and oxygen atoms, and Los Alamos ECP plus DZ (LANL2DZ) for tin and lead.17 All geometrical parameters were fully optimized and the harmonic vibrational frequencies were obtained analytically at the optimized structures. Additional B3LYP density functional calculations were done for comparison. Results Infrared spectra of products formed in the reactions of laserablated lead and tin atoms with H2O2 or H2 and O2 mixtures in excess argon during condensation at 10 K will be presented in turn. Theoretical calculations were performed to support the identifications of new tin and lead hydroxide molecules. The metal oxides and hydrides produced in these experiments have been assigned in previous studies.8,9 Common species generated by the laser-ablation process and trapped in solid argon, such as O3, O4-, HO2, and ArnH+ have been discussed in previous papers.18-23 Pb + H2O2. Infrared spectra of laser-ablated lead atoms with H2O2 in excess argon at 10 K, with annealing and irradiation afterward, (Figure 1) show the following spectral features: (i)

10.1021/jp053420p CCC: $30.25 © 2005 American Chemical Society Published on Web 09/21/2005

9014 J. Phys. Chem. A, Vol. 109, No. 40, 2005

Wang and Andrews

Figure 1. Infrared spectra in selected regions for laser-ablated lead atom reaction products with hydrogen peroxide in excess argon at 10 K: (a) spectrum of Pb and H2O2 codeposited for 60 min; (b) spectrum after annealing to 24 K; (c) spectrum after >220 nm irradiation, (d) spectrum after annealing to 34 K, (e) spectrum of Pb + H2 +O2 codeposited for 60 min, after ultraviolet irradiation and annealing to 22 K, (f) spectrum of Pb and D2O2 codeposited for 60 min, (g) spectrum after annealing to 24 K, (h) spectrum after >220 nm irradiation, (i) spectrum after annealing to 34 K, and (j) spectrum of Pb + D2 +O2 codeposited for 60 min, after ultraviolet irradiation and annealing to 22 K. The label C denotes the HOH‚‚‚O complex (see text) and HPC indicates an aggregate species common to laser-ablation experiments with hydrogen peroxide.

Figure 2. Infrared spectra in the O-H and Pb-O stretching regions for Pb and H2 + O2 reaction products in excess argon at 10 K: (a) spectrum of Pb + H2(6%) + O2(0.4%) codeposited for 60 min, (b) spectrum after 240-380 nm irradiation, (c) spectrum after >220 nm irradiation, (d) spectrum after annealing to 20 K, (e) spectrum of Pb + H2(6%) + 16,18O2[20% 16O2 + 50% 16O18O + 30% 18O2](0.4%), (f) after 240-380 nm irradiation, (g) spectrum after >220 nm irradiation, (h) spectrum after annealing to 20 K, (i) spectrum of Pb + H2(6%) + 18 O2(0.4%) codeposited for 60 min, (j) spectrum after 240-380 nm irradiation, (k) spectrum after >220 nm irradiation, and (l) spectrum after annealing to 20 K.

TABLE 1: New Infrared Absorptions (cm-1) Produced on Reactions of Pb + H2O2 and Pb + H2 + O2 in Excess Argon H2O2 O2, H2

16

3640.2 3609.3 3607.5 899.3 706.8 556.6 552.5 533.5 506.1 502.7

16,18

O2, H2

3640.2, 3629.0 3609.3, 3598.1

18

O2, H2

D2O2 O2, D2 identification

16

3628.9 3598.0

2685.4 2661.9 2660.4 671.8 545.9 555.4

533.7, 530.8, 517.2, 507.0

507.0

509.0

502.9, 492.5, 480.0, 477.6

477.6

487.3

Pb(OH)2 Pb(OH)2 Pb(OH)4 Pb(OH)4 Pb(OH)2 Pb(OH)4 Pb(OH)2 Pb(OH)2 Pb(OH) Pb(OH)2

On deposition absorptions at 502.7 and 533.5 cm-1 (stronger) and 552.5 and 706.8 cm-1 (weaker) track with two equal intensity bands at 3609.3 and 3640.2 cm-1 (group d) are observed. With D2O2, these bands shift to 487.3, 509.0, 545.9, 596.2, 2685.3, and 2693.8 cm-1, respectively (Table 1). On annealing to 20 and 24 K the intensities of group d bands doubled, and they increased 10% on full arc >220 nm irradiation. (ii) Weak group t bands at 556.6, 899.3, and 3607.5 cm-1 on deposition increased markedly on ultraviolet irradiation. With D2O2 the bands shifted to 555.4, 671.8, and 2684.6 cm-1, respectively. (iii) A weak group m shoulder absorption observed at 506.1 cm-1 beside the 502.7 cm-1 band on deposition decreased on annealing but increased on broadband irradiation (note the absence of a shoulder on the 533.5 cm-1 band). In addition weak lead oxide absorptions were observed at 711.2 cm-1 (PbO), 764.8 cm-1 (PbO2), and 555.6, 466.4 cm-1 (Pb2O2) in agreement with previous work.8,9 These bands appeared on deposition and decreased on annealing. Finally, absorptions common to hydrogen peroxide experiments with laser-ablated metals (labeled C) appear at 3730 and 3633 cm-1 for the HOH‚‚‚O complex identified previously.13,16 Pb + H2 + O2. Complementary experiments were done with Pb/O2/H2 in excess argon. On deposition stronger lead oxide bands and weaker lead hydride bands for PbH (1472.3 cm-1) and PbH2 (1532.7 cm-1) were observed.9b In addition broader

Figure 3. Infrared spectra in selected regions for laser-ablated tin atom reaction products with hydrogen peroxide in excess argon at 10 K: (a) spectrum of Sn and H2O2 codeposited for 60 min, (b) spectrum after >220 nm irradiation, (c) spectrum after annealing to 26 K, (d) spectrum after second >220 nm irradiation, (e) spectrum of Sn + H2 +O2 codeposited for 60 min, after ultraviolet irradiation and annealing to 22 K, (f) spectrum of Sn and D2O2 codeposited for 60 min, (g) spectrum after 240-380 nm irradiation, (h) spectrum after >220 nm irradiation, (i) spectrum after annealing to 24 K, (j) spectrum after annealing to 28 K, and (k) spectrum of Sn + D2 +O2 codeposited for 60 min, after ultraviolet irradiation and annealing to 22 K. The label C denotes the HOH‚‚‚O complex (see text) and HPC indicates an aggregate species common to laser-ablation experiments with hydrogen peroxide.

group d product bands appeared very weakly. On >380 nm irradiation, the PbH2 band disappeared and group d product bands increased slightly. Further ultraviolet irradiation increased group d bands markedly as well as lead oxide bands. The group d bands were observed within 0.3 cm-1of the positions using H2O2, but the bands due to group t and m were not observed in this experiment (shoulders are observed on both broader group d bands). One spectrum after ultraviolet irradiation and annealing is compared in Figure 1e. With Pb + D2 + O2, the group d product bands in Figure 1j show the same shifts as with D2O2. In addition, we observed lead deuteride bands for PbD (1055.3 cm-1) and PbD2 (1099.3 cm-1) and the same lead oxide

Infrared Spectra of M(OH)1,2,4

J. Phys. Chem. A, Vol. 109, No. 40, 2005 9015

TABLE 2: New Infrared Absorptions (cm-1) Produced on Reactions of Sn + H2O2 and Sn + H2 + O2 in Excess Argon H2O2 O2, H2

16

16,18

18

O2, H2

3659.8 3656.3 3625.1 843.3 727.4 646.5 640.0 631.9 598.8

D2O2 O2, D2

16

16

O2, H2

O2, HD

O2, D2

3660.0, 3684.5

3648.5

3660.3, 3625.1

3625.1, 3613.7

3613.7

2699.6, 2673.1

2699.7 2696.0 2673.5 656.9, 646.1 646.1

2683.3

identification

599.5, 596.7 580.7, 570.2

570.5

601

597.3

571.6

Sn(OH)2 Sn(OH)4 Sn(OH)2 Sn(OH)4 Sn(OH)2 Sn(OH)4 Sn(OH)2 Sn(OH)4 Sn(OH)2

566.2, 556.1 541.8, 539.3

539.1

569

567.6 575.7

531.2

Sn(OH) Sn(OH)2

724.2

569.5 565.7

18

2657.2

TABLE 3: Calculated Frequencies for M(OH) and HMO (M ) Pb, Sn) at the MP2 Level of Theory and Comparison with Observed of Frequencies Pb(OH) calcda 3899.8 (83) 641.6 (40) 512.3 (144)

Pb(OD) obsdb

calcda

506.1

2839.7 (56) 462.9 (5) 511.3 (161)

Sn(OH) calcda 3916.4 (92) 655.0 (31) 576.8 (169) a

Frequencies, cm

-1

Pb(18OH) calcda

HPbO calcda

assignment

3886.9 (80) 639.9 (38) 485.5 (131)

[1608.6 (806)] 609.9 (44) 489.4 (115)

O-H [Pb-H] stretching Pb-O-H bending Pb-O stretching

obsdb

Sn(18OH) calcd

HSnO calcd

assignment

577.8

3903.4 (89) 653.1 (34) 548.2 (151)

[1765.1 (456)] 564.7 (26) 515.9 (66)

O-H [Sn-H] stretching Sn-O-H bending Sn-O stretching

obsdb

Sn(OD) obsdb

calcda

569.5

2852.0 (60) 475.3 (29) 574.4 (146)

(intensities, km/mol). Argon matrix, cm-1. b

bands.9a,b A sample of 18O2 + H2 gave 18O shifts as listed in Table 1. With scrambled isotopic O2 (20% 16O2 + 50% 16O18O + 30% 18O2) the two lower bands for group d exhibited quartet distributions while the upper two bands revealed a doublet band. Sn + H2O2. As shown in Figure 3 laser-ablated Sn atom reactions with H2O2 in excess argon gave new absorptions at 565.7, 598.8, 640.0, 727.4, 3625.1, and 3656.3 cm-1 (group d), at 631.9, 646.5, 843.3, and 3618.7 cm-1 (group t), and at 569.5 cm-1 (group m). On annealing to 20 K, all product bands increased slightly, and on broadband irradiation, group d bands increased by 20% and group t bands by 150%, but group m changed very little. Further annealing sharpened group d and t bands but destroyed group m bands. Experiments with D2O2 gave large red shifts for all upper bands and small shifts for all lower bands, and the results are listed in Table 2. Weak tin oxide bands were observed (SnO, 811.7 cm-1; SnO2, 867.1 cm-1; Sn2O2, 610.7, 521.5 cm-1 in solid argon). Additional Sn + H2O2 experiments were done under different conditions. Higher laser energy and laser plume irradiation markedly increased the t bands relative to the d bands. Next, the H2O2 sample tube was surrounded with ice and the H2O2 precursor absorption intensities were reduced 10-fold: A comparable Sn experiment resulted in an 8-fold reduction of the strong t absorption at 3656.3 cm-1 relative to d at 3659.8 cm-1. The six d absorptions were observed with reduced intensity, but the 646.5 cm-1 t band was not detected. Sn + H2 + O2. Experiments with Sn and H2 + O2 reproduced the new bands as observed with Sn + H2O2, but the bands were broadened and shifted slightly. The tin oxide absorptions were stronger with weaker tin hydrides (SnH, 1641.7 cm-1; SnH2, 1657.0, 1654.8 cm-1).9,24 These bands appeared on deposition and increased markedly on broadband photolysis. The deuterium counterparts were observed accordingly using Sn + D2 + O2 sample. Tin reactions were done with HD + O2, and the results

Figure 4. Infrared spectra in the O-H, O-D, and Sn-O stretching regions for Sn and H2 + O2 reaction products in excess argon at 10 K: (a) spectrum of Sn + H2(6%) + O2(0.4%) codeposited for 60 min, (b) spectrum after 240-380 nm irradiation, (c) spectrum after >220 nm irradiation, (d) spectrum after annealing to 20 K, (e) spectrum of Sn + HD(6%) + O2(0.4% O2), (f) spectrum after >220 nm irradiation, (g) spectrum after annealing to 16 K, (h) spectrum of Sn + D2(6%) + O2(0.4%) codeposited for 60 min, (i) spectrum after >220 nm irradiation, and (j) spectrum after second >220 nm irradiation.

are shown in Figure 4. Additional experiments with 18O2 and 16,18O (scrambled) gave shifted bands, and the absorptions are 2 listed in Table 2. Calculations. Lead and tin mono-, di-, and tetrahydroxides were calculated at MP2 and B3LYP levels of theory to assist in new product identification. Computations for Pb(OH) gave PbOH and HPbO conformers; however, PbOH is 72 kcal/mol lower in energy than HPbO at the MP2 level. The analogous SnOH molecule is computed to be 55 kcal/mol lower than

9016 J. Phys. Chem. A, Vol. 109, No. 40, 2005

Wang and Andrews

TABLE 4: Calculated Frequencies for Pb(OH)2 (Cs), Pb(OH)2 (C2W), and HPbO(OH) at the MP2 Level of Theory and Comparison with Observed Frequencies Pb(OH)2 (Cs)

Pb(OD)2 (Cs)

Pb(18OH)2 (Cs)

calcda

obsdb

calcda

obsdb

calcda

obsdb

3905.3 (58) 3861.1 (60) 736.1 (40) 646.0 (52) 531.5 (111) 501.7 (144) 460.7 (74) 343.4 (136) 166.1 (13)

3640.2 3609.3 706.8 552.5 533.5 502.7

2843.3 (43) 2810.2 (42) 557.6 (81) 524.4 (113) 497.7 (113) 466.5 (7) 340.8 (48) 254.1 (76) 154.3 (11)

2693.8 2685.3 596.2 545.9 509.0 487.3

3892.5 (56) 3848.5 (57) 732.0 (37) 634.2 (48) 503.9 (102) 475.7 (131) 458.3 (71) 341.6 (131) 158.6 (11)

3628.9 3598.0

a

507.0 477.6

Pb(OH)2 (C2V) calcda

HPbO(OH) calcda

3900.3 (a1, 75) 3899.0 (b2, 11) 691.6 (a1, 1) 560.9 (b2, 81) 517.9 (a1,98) 496.1 (b2, 174) 385.8 (b1, 241) 293.3 (a2, 0) 176.6 (a1, 21)

3868.3 (79) [1933.7 (106)] 843.9 (65) 808.5 (121) 555.6 (23) 503.4 (116) 467.5 (19) 360.9 (109) 162.7 (55)

approximately mode O-H stretching O-H stretching [Pb-H] Pb-O-H bending Pb-O-H bending Pb-O stretching Pb-O stretching

Frequencies, cm-1 (intensities, km/mol). b Argon matrix, cm-1.

TABLE 5: Calculated Frequencies for Sn(OH)2 (Cs), Sn(OH)2 (C2W), and HSnO(OH) at the MP2 Level of Theory and Comparison with Observed Frequencies Sn(OH)2 (Cs)

Sn(OD)2 (Cs)

Sn(18OH)2 (Cs)

calcda

obsdb

calcda

obsdb

calcda

obsdb

3902.9 (66) 3857.4 (57) 782.3 (43) 697.5 (69) 573.9 (115) 541.6 (146) 470.6 (70) 359.4 (132) 182.0 (15)

3660.0 3625.1 727.4 640.0 598.8 565.7

2841.6 (46) 2807.4 (40) 573.3 (93) 567.5 (118) 535.9 (114) 504.9 (6) 348.3 (45) 265.9 (72) 169.1 (13)

2696.0 2673.0 646.1 597.3 575.7

3890.1 (63) 3844.8 (55) 778.0 (40) 694.5 (64) 545.7 (107) 515.1 (135) 468.2 (68) 357.5 (130) 174.0 (14)

3648.5 3613.7 724.2

a

570.5 539.1

Sn(OH)2 (C2V) calcd a

HSnO(OH) calcda

3894.6 (a1, 5) 3894.3 (b2, 138) 762.7 (a1, 80) 709.6 (b2, 58) 569.2 (a1, 102) 551.0 (b2, 167) 423.1 (b1, 172) 355.4 (a2, 0) 200.3 (a1, 1)

3898.4 (93) [2013.5 (104)] 839.9 (32) 823.8 (130) 624.2 (112) 601.1 (112) 461.5 (14) 348.5 (140) 189.8 (58)

approx mode O-H stretching O-H stretching [Sn-H] Sn-O-H bending Sn-O-H bending Sn-O stretching Sn-O stretching

Frequencies, cm-1 (intensities, km/mol). b Argon matrix, cm-1.

HSnO. The calculated frequencies are given in Table 3. Our B3LYP calculations gave similar results. A previous combined B3LYP/CCSD investigation found PbOH to be 54 kcal/mol lower energy than HPbO.6 For [Pb(OH)2] three different conformers, Pb(OH)2 (Cs), Pb(OH)2 (C2V), and HPbO(OH) were calculated, and the Cs and C2V structures are very close in energy while HPbO(OH) is 79 kcal/mol higher. Note that the Cs structure is stabilized on the potential energy surface by a weak internal hydrogen bond. At the MP2 level, we use 6-311++(d,p) for H and O and find a Ha-Ob distance of 2.777 Å, which is reduced to 2.714 Å with the large 6-311++G(3df,3pd) basis set. Apparently polarization and diffuse functions for H and O atoms are critical to describe the hydrogen bond. As we will discuss later, the calculated frequencies match experimental values much better using large basis sets. Similar calculations for Sn(OH)2 and at the MP2 level find that Sn(OH)2 lies lowest in energy while Sn(OH)2 (C2V) is 2.8 kcal/mol higher and HSnO(OH) is 40.0 kcal/mol higher in energy. The weak Ha-Ob hydrogen bond is shorter, 2.664 Å, for the tin dihydroxide. The calculated frequencies are listed in Tables 4 and 5. The B3LYP density functional gave comparable structures for the dihydroxides of Sn and Pb. For Sn(OH)2, the B3LYP functional found 3% shorter Sn-O bond lengths and other parameters almost the same. For Pb(OH)2, the computed structures were almost identical, and the B3LYP frequencies were slightly lower as expected.26 Calculations were also done for Pb(OH)4 and Sn(OH)4, and stable S4 symmetry structures were converged instead of Td symmetry found for the Hf(OH)4 and Zr(OH)4 molecules.13d The structures and parameters are given in Figure 5, and frequencies are listed in Table 6 for the MP2 level calculations. The bent M-O-H bond angles obtained for all lead and tin hydroxides suggest that the molecules exhibit substantial

Figure 5. Structures computed for lead and tin hydroxides at the MP2/ 6-311++G(3df,3pd)/LANL2DZ level of theory. Bond lengths are given in angstroms and bond angles in degrees.

covalent bonding character.25 The B3LYP functional gave almost identical computed structures for both M(OH)4 molecules. Discussion The new lead and tin mono-, di-, and tetrahydroxides will be identified through the use of different reagents, photochemical properties, isotopic substitution, and comparison with theoretical frequency calculations. Pb(OH)2. Two strong new infrared absorptions at 533.5 and 502.7 cm-1 in the Pb-O stretching region and two new bands

Infrared Spectra of M(OH)1,2,4

J. Phys. Chem. A, Vol. 109, No. 40, 2005 9017

TABLE 6: Calculated Frequencies for Pb(OH)4 and Sn(OH)4 at the MP2 Level of Theory in S4 Symmetry and Comparison with Observed Frequencies Pb(OH)4 calcda 3881.5 (0) 3879.5 (44) 3879.2 (150 × 2) 878.6 (89 × 2) 876.8 (60) 861.3 (0) 565.6 (99 × 2) 556.7 (99) 542.8 (0) 261.0 (176) 227.0 (48 × 2) 178.7 (24) 163.9 (0) 162.7 (23 × 2) 129.7 (0) 92.5 (57) b

Pb(OD)4 obsdb

3607.5 899.3 556.6

calcda 2826.5 (0) 2823.8 (28) 2823.7 (99 × 2) 653.5 (77 × 2) 652.6 (63) 625.7 (0) 555.9 (75 × 2) 545.8 (65) 536.9 (0) 210.5 (117) 175.9 (51 × 2) 168.7 (6) 129.6 (0) 146.6 (3 × 2) 116.4 (0) 81.5 (48)

Sn(OH)4 obsdb

2684.6 671.8 555.4

calcda

Sn(OD)4 obsdb

3928.5 (0) 3927.2 (56) 3926.6 (144 × 2) 822.8 (115 × 2) 815.7 (55) 802.0 (0) 638.7 (136 × 2) 626.9 (127) 610.5 (0) 275.2 (169) 236.6 (66 × 2) 215.5 (29) 173.5 (0) 193.1 (12 × 2) 151.4 (0) 101.6 (104)

3656.3 843.3 646.5 631.9

calcda 2861.2 (0) 2858.8 (37) 2858.6 (96 × 2) 610.4 (35 × 2) 602.7 (6) 606.7 (0) 636.3 (165 × 2) 624.4 (139) 580.8 (0) 233.8 (119) 199.3 (57 × 2) 199.4 (1) 57.7 (0) 159.3 (2 × 2) 118.0 (0) 86.3 (80)

obsdb

2696.0

654.4 650.9

approx mode O-H stretching O-H stretching O-H stretching M-O-H bending M-O-H bending M-O-H bending M-O stretching M-O stretching M-O stretching

a Frequencies, cm-1 (intensities, km/mol). The a mode has 0 intensity and the b mode has the given intensity, and the e mode has ×2 intensity. Argon matrix, cm-1.

at 3640.2 and 3609.3 cm-1 in the O-H stretching region labeled d track together in the Pb + H2O2 experiments. In the reaction with D2O2, the Pb-O bands shift to 509.0 and 487.3 cm-1 and the O-H bands to 2685.4 and 2661.9 cm-1, respectively. The red-shift of Pb-O stretching modes with deuterium substitution indicates that these two modes are coupled to the H atom motion. The Pb-O stretching frequencies of PbO and PbO2 in solid argon at 711.2 and 764.8 cm-1 show considerable differences in PbO bonding for the new product molecule. Complementary experiments with Pb + H2 + O2 gave slightly broader bands centered at 533.5 and 502.7 cm-1 and at 3640.2 and 3609.3 cm-1, which shift to 507.0 and 477.6 cm-1 and to 3628.9 and 3598.0 cm-1 with Pb + H2 + 18O2. The isotopic frequency ratios for the upper bands (H/D, 1.3556, 1.3559 and 16/18, 1.00311, 1.00314) are characteristic of O-H stretching modes, and the 16/18 ratios for the lower bands (1.0523, 1.0526) describe an antisymmetric O-Pb-O stretching mode (the ratios for PbO2 (1.0513) and PbO (1.0561) are distinctly different).9a The isotopic frequency patterns with 16O2 + 16,18O2 +18O2 (1: 2:1) show quartet distributions for the two lower modes (Figure 2), demonstrating that two inequivalent oxygen atoms are involved. However, for the two upper modes two doublets were observed at 3640.4, 3629.0 cm-1 and at 3609.3 and 3598.1 cm-1, which indicate very little or no coupling between two inequivalent O-H groups. Therefore, the Pb(OH)2 molecule with two inequivalent O-H subunits is identified, and the group d bands are assigned accordingly. This is distinctively different from group 2, 4, and 12 dihydroxides, which exhibited one strong antisymmetric stretching mode in the O-H and M-O stretching regions for these M(OH)2 molecules with two equivalent O-H subunits.13 The identification of Pb(OH)2 is confirmed by MP2 frequency calculations (Table 4). The planar Cs structure at the MP2 level gives two strong Pb-O stretching modes calculated at 531.5 and 501.7 cm-1, which are in excellent agreement with the 533.5 and 502.7 cm-1 observed values. The two modes are predicted to red-shift by 33.8 and 35.2 cm-1 for Pb(OD)2, which are larger than the experimental shifts of 24.5 and 15.4 cm-1. Obviously the theoretical calculation predicts more H atom coupling than is observed for the two modes. It is interesting to note that the 533.5 cm-1 band has a greater deuterium shift than the 502.7 cm-1 band, which tells us that the two modes couple with H (and D) atoms differently for this asymmetric structure. The

predicted 18O2 shifts are slightly more than experimental, but the calculated quartet isotopic patterns for each mode match experiment very well. With MP2 two O-H stretching modes are predicted at 3905.3 and 3861.1 cm-1 with almost the same intensities, which are overestimated by 8.2% and 6.1%, respectively, and are in line with expectations.26 Meanwhile the calculation reveals identical O-H stretching modes for O atoms substituted by 18O2 or 16,18O2, showing no coupling between the two O-H stretching modes. The two sharp weaker group d bands at 706.8 and 552.5 cm-1 are 4 and 14% below the calculated O-H bending frequencies. Clearly, the free O-H bending mode is described better by the MP2 calculation that the O-H group involved in the weak intramolecular hydrogen bond. The 706.8 cm-1 band shifts to 545.9 cm-1 with D2O2 (H/D ratio 1.2947) and gains considerable intensity from mixing with Pb-O stretching modes. The deuterium counterpart for the weaker 552.5 cm-1 band is not detected. Calculations for Pb(OH)2 with the Cs structure using B3LYP give slightly lower Pb-O stretching modes at 523.4 and 494.5 cm-1 and O-H stretching modes at 3862.6 and 3825.3 cm-1. However predicted deuterium shifts are only 2-3 cm-1 for both Pb-O stretching modes, meaning that the B3LYP calculation does not describe these modes as well as MP2. Our B3LYP calculation found for the optimized structure a Hb-Oa distance 0.043 Å longer than the MP2 value, and as a result the H atom coupling is underestimated. On the other hand, frequencies calculated for the C2V structure (Table 4) have several shortcomings. The computed O-H stretching mode separation of 1.3 cm-1 for two equivalent bonds is far from the observed 30.9 cm-1 and Cs computed 44.2 cm-1 values, and the 21.8 cm-1computed Pb-O stretching mode separation is in poor agreement with the observed 30.8 cm-1 and Cs computed 29.8 cm-1 values. Finally, the C2V structure has only one bending mode with observable intensity and the Cs structure has two in good agreement with the observed absorptions. The insertion reaction of Pb into H2O2 is exothermic by 148 kcal/mol (MP2) and 131 kcal/mol (B3LYP), as shown in reaction 1. Spontaneous insertion reaction is observed on annealing of Pb + H2O2 suggesting that no activation energy is required. Similar insertion reactions were observed for group 2, 4,and 12 metals.13 For the reaction of Pb + O2 + H2, we

9018 J. Phys. Chem. A, Vol. 109, No. 40, 2005

Wang and Andrews

propose that OPbO is formed first on photolysis, and then reacts further with H2 in the solid matrix cage to give Pb(OH)2. Reaction 2b is 105 kcal/mol exothermic (MP2 level).

Pb + H2O2 f Pb(OH)2 (∆E ) -148 kcal/mol)

(1)

Pb + O2 f OPbO (∆E ) -74 kcal/mol)

(2a)

OPbO + H2 f Pb(OH)2 (∆E ) -105 kcal/mol)

(2b)

Pb(OH)4. Three new infrared absorptions at 556.6, 899.3, and 3607.5 cm-1 labeled t in Pb + H2O2 experiments appeared very weakly on deposition and increased upon sample irradiation and annealing, but these bands were not observed with the hydrogen/oxygen reagent. With Pb + D2O2 these bands shifted to 555.4, 671.8, and 2660.4 cm-1, giving H/D isotopic ratios of 1.0022, 1.3386, and 1.3560, respectively. Apparently the 556.6 cm-1 band has a very small deuterium shift, and it can be assigned to the Pb-O stretching mode. The 899.3 and 3607.5 cm-1 bands have large deuterium shifts, which are due to PbO-H bending and O-H stretching modes, respectively. These group t bands are appropriate for Pb(OH)4. Comparison with frequencies calculated by MP2 and B3LYP show very good agreement with experimental values. Absorptions of Pb(OH)4 are predicted by B3LYP calculation to give a stronger degenerate Pb-O stretching mode at 553.1 cm-1, Pb-O-H bending mode at 880.6 cm-1, and O-H stretching mode at 3837.3 cm-1, and counterparts of Pb(OD)4 are found at 543.2 (Pb-O stretching), 655.5 (Pb-O-H bending), and 2793.3 cm-1 (O-H stretching). Our MP2 calculation gives slightly higher frequencies (Table 6). Unfortunately in Pb + H2 + O2 experiments we did not observe these Pb(OH)4 absorptions because of the different reaction mechanism, and no 18O and 16,18O isotopic frequencies were observed to confirm the assignment. Formation of Pb(OH)4 must come from the reaction of Pb(OH)2 and H2O2 (reaction 3), which is exothermic by 65 kcal/ mol (MP2) and 49 kcal/mol (B3LYP). Notice that reaction 3 is less than half as exothermic as reaction 1. Lead in the tetravalent state is destabilized in part because of the relativistic effect, which increases s orbital ionization energy.10,11,27 As a result the bond strength is reduced for Pb(IV) species.28,29 The Pb(IV) halides are much less favorable than Pb(II) halides.10 The yield of Pb(OH)4 increases significantly on photolysis but barely on annealing, implying much less exothermic reaction. Although no absorption of Pb(OH)4 is detected in the Pb + O2 + H2 experiment, this is significantly different from Zr and Hf, where the straightforward reactions produce major products, Zr(OH)4 and Hf(OH)4 based on very large computed exothermic reaction energies.13d

Pb(OH)2 + H2O2 f Pb(OH)4 (∆E ) -65 kcal/mol) (3) The relative stability of Pb(OH)4 and Pb(OH)2 is expected to be comparable to that observed for the chlorides and fluorides, which is better than for the recently described case for PbH4 and PbH2.9c Although the PbH2 + H2 f PbH4 reaction is slightly endothermic,30,31 PbH4 has been formed from PbH2 in solid hydrogen.9c The corresponding reaction 3 is exothermic, which suggests more stability for Pb(OH)4 than for PbH4. Pb(OH). A weak absorption at 506.1 cm-1 in Pb + H2O2 experiments can be tentatively assigned to the Pb-O stretching mode of Pb(OH) based on annealing and photochemical behavior. This species is reduced quickly on annealing but is regenerated on ultraviolet irradiation, and is decreased on further annealing like the behavior of an active radical. The weaker O-H stretching fundamental is probably obscured by the nearby

mode for Pb(OH)2. Our MP2 calculation for Pb(OH) predicts the Pb-O stretching mode at 512.3 cm-1, which is in good agreement with the 506.1 cm-1 band. Our B3LYP calculation gives a very similar result. The insertion product HPbO is not observed here because of its very high energy relative to PbOH. Unfortunately, the Pb(OD) counterpart is not observed due to lower product yield. Energetic laser-ablated lead atoms react with H2O2 to give excited Pb(OH)2 that can either be trapped in the solid matrix or decomposed to Pb(OH) + OH. On photolysis and further annealing the Pb(OH) absorptions decrease, possibly reacting with OH to give Pb(OH)2. Reaction 4 is exothermic but not nearly as much so as reaction 1.

Pb + H2O2 f [Pb(OH)2]* f Pb(OH) + OH (∆E ) -43 kcal/mol) (4) Sn(OH)2. The group d bands at 565.7, 598.8, 640.0, 727.4, 3625.1, and 3660.0 cm-1 track together in experiments with Sn + H2O2 upon annealing, irradiation, and changes in hydrogen peroxide concentration, and they can be assigned and to Sn(OH)2. The same absorptions were found in Sn + H2 + O2 reactions but with slightly lower intensities. The 565.7 and 598.8 cm-1 bands shift to 539.1 and 570.5 cm-1 with 18O2 substitution, suggesting two Sn-O stretching modes, while the 640.0 and 727.4 cm-1 bands show very small shifts, which are appropriate for the Sn-O-H bending modes. The 3660.0 and 3625.1 cm-1 bands exhibited 11.5 and 11.4 cm-1 red-shifts in the Sn + H2 + 18O2 experiment, which are typical for O-H stretching modes in a metal hydroxide. The oxygen isotopic quartet pattern for both 565.7 and 598.8 cm-1 bands and doublets for the 3660.0 and 3625.1 cm-1 bands were observed with 16O2 + 16O18O + 18O + H , showing two inequivalent oxygen atoms, which are 2 2 very similar that found for Pb(OH)2. With D2O2 the 598.8 cm-1 band shifts red only 1.5 cm-1 to 597.3 cm-1, while the 565.7 cm-1 band shifts blue by 10.0 cm-1, indicating different couplings of the O-D bending vibrations with these modes. This is not the case for Pb(OH)2, for which both Pb-O modes shift red with deuterium substitution. The O-D stretching modes appeared at 2699.7 and 2673.5 cm-1, giving 1.3556 and 1.3559 H/D ratios, which again are typical for metal dihydroxides.13 The O-D stretching modes red shifted 16.4 and 16.3 cm-1 with Sn + D2 + 18O2, which is in line with previous observations.13 The Sn-18OD stretching modes were observed at 571.6 and 531.2 cm-1. The upper band exhibits a reasonable 18O shift, but the lower band appears to have a considerably different amount of mixing with the Sn-O-D bending mode for the blue shift described above. The vibrational assignment of Sn(OH)2 is supported by theoretical calculations. Our MP2 calculation shows that two structures, namely Cs and C2V, are very close in energy but very different in frequencies (Table 5). The calculated frequencies of the Cs structure predict the experimental frequencies satisfactorily, but the C2V frequencies suffer from the mode separation deficiencies described above for lead dihydroxide. These are Sn-O stretching modes at 573.9 and 541.6 cm-1, O-H stretching modes at 3902.9 and 3857.4 cm-1, which must be scaled down by 6.2 and 6.0%, respectively, and Sn-O-H bending modes at 782.3 and 697.5 cm-1. It is not surprising that the two predicted quartets for two Sn-O stretching modes and two doublets for O-H stretching modes with 16O and 18O substitution match experimental bands very well. The predicted O-D stretching frequencies at 2841.6 and 2807.4 cm-1 are in reasonable agreement (scale down by 5.0 and 4.7%), but the Sn-O stretching modes are predicted to shift down 6.4 and

Infrared Spectra of M(OH)1,2,4

J. Phys. Chem. A, Vol. 109, No. 40, 2005 9019

5.7 cm-1. Apparently the O-H coupling effect for these modes is underestimated. The upper O-H(D) stretching mode is due to the free O-H(D) bond and the lower mode to O-H(D) in the intramolecular hydrogen bond. The Sn(OH)2 molecule is formed spontaneously from combination of Sn and H2O2 (reaction 5), which is exothermic by 154 kcal/mol (MP2) and 145 kcal/mol (B3LYP) and is slightly more exothermic than that found in the lead reaction. Formation of Sn(OH)2 is also through reaction of Sn with H2 and O2 (reaction 6), and ultraviolet irradiation promoted this reaction significantly.

Sn + H2O2 f Sn(OH)2 (∆E ) -154 kcal/mol)

(5)

Sn +O2 f OSnO (∆E ) -106 kcal/mol)

(6a)

OSnO + H2 f Sn(OH)2 (∆E ) -79 kcal/mol) (6b) [We note that the difference in the energies of reactions 1 and 2 compared to reactions 5 and 6, namely -31 kcal/mol, is in fact the energy of formation of hydrogen peroxide computed at the MP2 level, -31 kcal/mol.] The Sn, HD, O2 reaction was done to examine the distribution of isomers involving H or D in the intramolecular hydrogen bonding position, and the spectra are compared in Figure 4 with those from similar H2 and D2 experiments. This is analogous to HD and F2 reactions, which formed primarily the H-F‚‚‚D-F dimer.32 Our calculations show no coupling between the O-H(D) bonds so the frequencies are the same for both Sn(OH)(OD) isomers as in the Sn(OH)2 and Sn(OD)2 molecules. Note for the HD + O2 spectrum that the 3660 and 2673 cm-1 bands are 60 ( 10% stronger relative to the 3625 and 2700 cm-1 counterparts for Sn(OH)2 and Sn(OD)2. This means that the (HO)Sn(OD) isomer [D‚‚‚O] is favored over the (DO)Sn(OH) isomer [H‚‚‚O]. Computed zero point vibrational energies find the former 44 cal/mol more stable. Recall that these molecules are formed in reaction 6b with approximately 79 kcal/mol of excess internal energy. Sn(OH)4. The group t bands at 631.9, 646.5, 843.3, and 3656.3 cm-1 are assigned to Sn(OH)4 in the Sn + H2O2 experiment. Apparently the 631.9 and 646.5 cm-1 bands are due to Sn-O stretching modes and the 3656.3 cm-1 band is due to the O-H stretching mode based on similar bond properties with Sn(OH)2. The deuterium counterparts were found at 650.9, 654.4, and 2696.0 cm-1 with Sn + D2O2. Here both Sn-O modes show blue shifts suggesting strong O-D coupling, since the 843.3 cm -1 Sn-O-H bending mode shifts into this region on deuterium substitution as found for the same mode in Sn(OD)2. The Sn(OH)4 molecule is calculated to have S4 symmetry at MP2 level, and the Sn-O stretching mode splits into an active b component at 626.9 cm-1 and a degenerate e mode at 638.7 cm-1, which underestimate the observed values by 5.0 and 7.8 cm-1. The Sn-O stretching modes in Sn(OD)4 are predicted at 624.4 and 636.3 cm-1, respectively, which unfortunately deviate from experimental values. Again the density functional theoretical calculation does not describe the O-H coupling correctly. The predicted b and e O-H stretching modes are too close to resolve. The stronger e mode for Sn(OH)4 at 3926.6 cm-1 and O-D stretching mode for Sn(OD)4 at 2858.6 cm-1 must scale 6.9 and 5.7% to fit the observed 3656.3 and 2696.0 cm-1 bands. Our B3LYP calculation gave very similar results. The stable Sn(OH)4 molecule is formed by further reaction of Sn(OH)2 with H2O2 (reaction 7). This reaction is exothermic by 123 kcal/mol, which is almost twice as high as calculated

for the reaction to form Pb(OH)4. Notice that the yield of Sn(OH)4 is slightly higher and weak bands due to Sn(OH)4 were observed in Sn + H2 + O2 experiments, indicating Sb(OH)4 is more stable than Pb(OH)4. In fact only PbCl4 is observed experimentally while several SnX4 (X ) Cl, Br, I) molecules have been identified. Tetrahydroxide molecules have also been observed for group 4 metals in even more exothermic reactions with hydrogen peroxide in excess argon.13d

Sn(OH)2 + H2O2 f Sn(OH)4 (∆E ) -123 kcal/mol) (7) Sn(OH). A weak band at 569.5 cm-1 (Sn-O stretching) in Sn + H2O2 experiments is tentatively assigned to Sn(OH). With D2O2 the band shifts to 567.6 cm-1. These bands appear on sample irradiation along with Sn(OH)2 and Sn(OD)2 and then decrease on further annealing as expected for a reactive species. Our MP2 calculation predicts this Sn-O stretching mode at 576.8 cm-1 with a 2.4 cm-1 shift for Sn(OD) (Table 3), which are in good agreement. The weaker O-H stretching mode is probably masked by the nearby mode for Sn(OH)2. The formation of Sn(OH) is through reaction 8, and the product is then trapped in the low-temperature matrix.

Sn + H2O2 f [Sn(OH)2]* f Sn(OH) + OH (∆E ) -51 kcal/mol) (8) Bonding in Group 14 Metal Hydroxides. The Cs symmetry structures calculated and observed for Pb(OH)2 and Sn(OH)2 are unique: (i) the O-M-O bond is close to a right angle, which is very different from group 2 and transition metal hydroxides, (ii) a weak intramolecular hydrogen bond is formed that is not found in other metal hydroxides, and (iii) Pb(OH)4 and Sn(OH)4 are minor reaction products although Pb(IV) and Sn(IV) halides have been observed experimentally.10 First, MH2 and MX2 (M ) Pb, Sn, X ) halogen) triatomic molecules favor bent structure with near 90° bond angles, which has been explained by pure p orbital bonding because of high s orbital ionization energy.10,11 However OPbO and OSnO molecules are linear based on matrix infrared spectra and theoretical calculations, which suggests that both s and p orbitals participate in this bonding interaction. Recall that group 2 metal hydroxides and oxides have very similar O-M-O bond angles since only two s electrons are involved in the bonding.13 Second, the Pb-O and Sn-O bonds in Pb(OH)2 and Sn(OH)2 have substantial covalent character. The O-H stretching frequencies are very near this mode in the most covalent metal hydroxides such as Cd(OH)2 and Hg(OH)2, (3659.3 and 3629.4 cm-1), but somewhat lower than O-H stretching frequencies in group 2 and group 4 metal hydroxides.13 Third, the smaller O-Pb-O bond angle provides a shorter distance between Ob and Ha, which attract each other giving extra stabilization energy from a weak intramolecular hydrogen bond. Only the Cs geometry is observed although the calculated energies of the Cs and C2V structures are very close. Finally, the intramolecular hydrogen bond computed here for Sn(OH)2 and Pb(OH)2 is weaker and substantially longer (2.7 Å) than that computed for the strong phenol-ammonia hydrogen bonded complexes (1.9 Å).33 The formation of tetrahydroxides of lead and tin is analogous to their tetrahalides. Theoretical calculations show that Sn(OH)4 is more stable than Pb(OH)4, which can be confirmed from experiments in that the yield of Sn(OH)4 is more than that of Pb(OH)4. Obviously relativistic effects play an important role here and cause the lead tetravalent state to be less favorable.10,11 The Sn(OH)4 and Pb(OH)4 molecules with S4 structures and bent M-O-H bonds are clearly more covalent25 than the Zr-

9020 J. Phys. Chem. A, Vol. 109, No. 40, 2005 (OH)4 and Hf(OH)4 molecules with Td structures.13d The O-H stretching frequencies have the same relationship described for the dihydroxides, namely the more covalent Sn and Pb tetrahydroxides have lower (3656.3 and 3607.5 cm-1) O-H modes than the more ionic Zr and Hf tetrahydroxides (3782.6 and 3796.4 cm-1). Conclusions Laser-ablated lead and tin atoms react with H2O2 to form the hydroxides M(OH), M(OH)2, and M(OH)4 (M ) Pb, Sn), which were observed in infrared spectra after sample condensation in solid argon. The major M(OH)2 product was also produced from H2 and O2 mixtures, which allowed 18O2 substitution. The band assignments were confirmed by appropriate D2O2, D2, 16O18O, and 18O2 isotopic shifts. Complementary MP2 and B3LYP calculations provided molecular structures and vibrational frequencies to aid in assignment of the infrared spectra. The minimum energy structure found for M(OH)2 has Cs symmetry and a weak intramolecular hydrogen bond. In experiments with Sn, HD, and O2, the internal D bond is favored over the internal H bond for Sn(OH)(OD). Calculations for the Pb(OH)4 and Sn(OH)4 molecules find S4 symmetry and substantial covalent character, and relativistic effects limit the yield of the less stable lead tetrahydroxide molecule. Acknowledgment. We appreciate financial support from NSF Grant CHE 03-53487. References and Notes (1) Sterckeman, T.; Douay, F.; Proix, N.; Fourrier, H. EnViron. Pollut. 2000, 107, 377. (2) (a) Abbott, M. B.; Wolfe, A. D. Science 2003, 301, 1893. (b) Shotyk, W.; Weiss, D.; Appleby, P. G.; Cheburkin, A. K. Frei, R.; Gloor, M.; Kramers, J. D.; Reese, S.; Van Der Knaap, W. O. Science 1998, 281, 1635. (3) Carignan, J.; Simonetti, A. Gariepy. C.Atmos. EnViron. 2002, 36, 3759. (4) Jain, N. B.; Laden, F. Guller, U.; Shankar, A.; Kazani, S.; Garshike, E. Am. J. Epidemiology 2005, 161, 968. (5) Endres, J.; Montgomery, J.; Welch, P. J. EnViron. Health 2002, 64, 20. (6) Benjelloun, A. T.; Daoudi, A.; Chermette, H. J. Chem. Phys. 2004, 121, 7207. (7) Ziebarth, K.; Beridhor, R.; Shestakov, O.; Fink, E. H. Chem. Phys. Lett. 1992, 190, 271. (8) (a) Ogden, J. S.; Ricks, M. J. J. Chem. Phys. 1972, 56, 1658 (PbO). (b) Bos, A.; Ogden, J. S. J. Phys. Chem. 1973, 77, 1513, (Sn + O2). (9) (a) Chertihin, G. V.; Andrews, L. J. Chem. Phys. 1996, 105, 2561, (Pb + O2). (b) Wang, X.; Andrews, L.; Chertihin, G. V.; Souter, P. F. J.

Wang and Andrews Phys. Chem. A 2002, 106, 7696, (Pb, Sn + H2). (c) Wang, X.; Andrews, L. J. Am. Chem. Soc. 2003, 125, 6581, (Pb + H2). (10) Haigittai, M. Chem. ReV. 2000, 100, 2233 and references therein. (11) Pyykko, P. Chem. ReV. 1988, 88, 563. (12) (a) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.: Bochmann, M. AdVanced Inorganic Chemistry, 6th ed.; Wiley: New York, 1999. (b) CRC Handbook, 66th ed.; CRC Press: Boca Raton, FL, 1985. (13) (a) Wang, X.; Andrews, L. J. Phys. Chem. A 2005, 109, 2782 (group 2 metal hydroxides). (b) Wang, X.; Andrews, L. J. Phys. Chem. A 2005, 109, 3849 (Zn, Cd hydroxides). (c) Wang, X.; Andrews, L. Inorg. Chem. 2005, 44, 108 (Hg(OH)2). (d) Wang, X.; Andrews, L. Inorg. Chem. 2005, 44, in press (hafnium metal hydroxides). Wang, X.; Andrews, L. J. Phys. Chem. A 2005, 109, in press (group 4 metal hydroxides). (14) Andrews, L. Chem. Soc. ReV. 2004, 33, 123 and references therein. (15) Pettersson, M.; Tuominen, S.; Rasanen, M. J. Phys. Chem. A 1997, 101, 1166. (16) Pehkonen, S.; Pettersson, M.; Lundell, J.; Khriachtchev, L.; Rasanen, M. J. Phys. Chem. A 1998, 102, 7643 and references therein. (17) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.6. (18) Wang, X.; Andrews, L. J. Phys. Chem. A 2001, 105, 5812. (19) Zhou, M.; Hacalogu, J.; Andrews, L. J. Chem. Phys. 1999, 110, 9450. (20) Milligan, D. E.; Jacox, M. E. J. Mol. Spectrosc. 1973, 46, 460. (21) Jacox, M. E.; Thompson, W. E. J. Chem. Phys. 1994, 100, 750. (22) Milligan, D. E.; Jacox, M. E. J. Chem. Phys. 1963, 38, 2627. (23) Smith, D. W.; Andrews, L.J. Chem. Phys. 1974, 60, 2627. (24) See ref 8(a) for Kr matrix observations. In solid argon, Sn16O peaked at 810.2 and Sn18O at 769.9 cm-1. The SnO2 bands gave resolved 116Sn, 118Sn, and 120Sn isotopic triplets at 870.2, 868.5, and 867.1 cm-1 for Sn16O , 2 at 854.2, 852.7, and 851.2 cm-1 for Sn16O18O, and at 831.7, 830.1, and 828.5 cm-1 for Sn18O2. (25) Ikeda, S.; Nakajiama, T.; Hirao, K. Mol. Phys. 2003, 101, 105. (26) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502. (27) Desclaux, J. P.; Pyykko, P. Chem. Phys. Lett. 1974, 29, 534. (28) Kaupp, M.; Schleyer, P. v. R. J. Am. Chem. Soc. 1993, 115, 1061. (29) Schwerdtfeger, P.; Heath, G. A.; Dolg, M.; Bennett, M. A. J. Am. Chem. Soc. 1992, 114, 7518. (30) Dyall, K. G.; Taylor, P. R.; Faegri, K.; Partridge. H. J. Chem. Phys. 1991, 95, 2583. (31) Dyall, K. G. J. Chem. Phys. 1992, 96, 1210. (32) Hunt, R. D.; Andrews, L. J. Chem. Phys. 1985, 82, 4442. (33) See for example: (a) Iwasaki, A.; Fujii, A.; Watanabe, T.; Ebata, T.; Mikami, N. J. Phys. Chem. 1996, 100, 16053. (b) Abkowicz-Bienko, A.; Latajka, Z. J. Phys. Chem. A 2000, 104, 1004.